1. Field of the Invention
The present invention relates to a galvanic isolation device for a modem, of the type comprising an optical coupler, specially adapted to ensure galvanic isolation between a modem of a portable micro-computer and a telephone line.
2. Description of the Prior Art
It is well known that, in the case in which equipment fed with current by the mains voltage is intended to be connected to the telephone network, the "Postes et Telecommunications" (PTT) regulations demand galvanic isolation between the equipment and the telephone network, for example resisting a voltage of 2500 volts, in order not to damage the telephone lines in the case in which a high voltage, due to a defect in the equipment, a storm, etc . . . appears in the circuits of the equipment.
Conventionally, in the case of a modem or modem card coupled to an office or portable computer, the galvanic isolation between the modem and the telephone line is achieved by means of a transformer. Such a component has the advantage of not consuming current and of having transfer characteristics which vary little in time. However, the transformers used, in order to retain good linearity, are necessarily bulky and heavy, which gives rise to housing problems when incorporating the modem in the computer, particularly in a compact or portable personal computer. The weight of a computer is also substantially increased. Furthermore, due to their inductive structure, transformers are very prone to high frequency interference.
Certain manufacturers have already attempted to overcome these disadvantages by producing galvanic isolation between the telephone network and the modem by means of optical couplers.
However, the use of an optical coupler gives rise to certain technical difficulties. Firstly, the transfer characteristic of an optical coupler can vary widely in time due to an ageing phenomenon, particularly of its light-emitting diode. Furthermore, in a same series of optical couplers, the value of the current gain of the photo-transistor varies greatly from one optical coupler to another, as it does generally in all transistors. Typically, this gain can vary between 100 and 400.
Because of these spreads, the value of the analog voltage applied to the telephone line therefore also varies greatly, which is unacceptable with respect to the PTT regulations.
A known solution to this disadvantage related to the spread of transfer characteristics of the optical couplers can consist in coupling each optical coupler with an adjustable-gain amplification stage, and in adjusting the gain of this stage, case by case, in order to compensate the response variations of the optical coupler. However, this solution is not compatible with the demands of mass production, as an individual adjustment of each modem or modem card is necessary.
Another solution could consist in combining an optical coupler with an automatic gain control circuit (AGC), widely used in radio receivers, particularly in amplitude modulation and frequency modulation receivers. This circuit would make it possible, by modifying the biasing of the light-emitting diode of the optical coupler, to give the complete circuit a constant gain.
Such a solution would however be very difficult to implement because, in order to retain galvanic isolation, the AGC voltage to be applied to the control circuit of the light-emitting diode should be carried out by another optical coupler, for example of the photo-diode type, which would affect the efficiency of the AGC and render the circuit more expensive. Furthermore, a reaction carried out not at the level of the light-emitting diode but in the photo-transistor emitter, would be capable of reducing the spread of gain between one optical coupler circuit and another, but not sufficiently for the considered practical application.
Furthermore, in the case of a photo-transistor, whose current gain would be extremely low, there would be a risk, due to an excessive photonic current, of bringing the photo-transistor into its saturated state, and of thus causing unacceptable distortions in the output signal. In fact, the range of linearity of an optical coupler photo-transistor is traditionally very narrow.
Furthermore, such a circuit should be the subject of the meticulous adaptations or adjustments in the case in which it would be required to use a different type of optical coupler.